The use of generally tubular-shaped prostheses (also referred to as stents or grafts) to treat vascular disease or injury is well known. A typical treatment involves implanting a prosthesis to replace and/or repair portions of damaged or diseased blood vessels. Such prostheses have been formed from both natural and synthetic materials. As between natural and synthetic materials, much attention has been focused upon the development and use of acceptable synthetic prostheses formed from materials such as polymers or the like.
Current clinical practice relating to vascular prostheses has focused on the development and use of porous structures. Porous structures are currently preferred as such porous structures, after implantation in a host, tend to become covered with a lining of thrombus. Thus, the surface of the structure exposed to blood flow i.e., the flow surface, becomes less thrombogenic over time. Accordingly, synthetic prostheses desirably exhibit a certain amount of porosity effective to help promote tissue ingrowth that results in the formation of a lining of thrombus. Porosity is desirable on both the inner and outer surfaces of a prosthesis, but is particularly desirable on the inner, or flow, surface.
The quality and quantity of the porosity of the inner surface of a synthetic prosthesis is highly dependent upon the manner in which the prosthesis is made. For example, some synthetic prostheses have been made from compositions comprising a biocompatible, water insoluble elastomeric resin and a water soluble salt. After a prosthesis is formed from such a composition, the salt is rinsed out using hot water. The voids in the prosthesis formerly occupied by the salt contribute to the porosity of the inner wall of the prosthesis. This approach has a number of drawbacks, however. First, the resultant pore shape tends to correspond to the crystalline shape of the salt, and therefore tends to have sharp edges and corners. These sharp edges and corners can act like stress concentrators from which stresses are easily propagated. This has a negative impact upon the mechanical strength of the prosthesis. Further, a relatively high concentration of salt is generally required to achieve desired levels of porosity, which also can result in a mechanically weak prosthesis.
Synthetic prostheses with some porosity have also been prepared using the so-called continuous fiber winding technique. According to this technique, a polymer melt, solution, or dispersion is extruded through a fine orifice to form a polymeric fiber. The resultant polymeric fiber is then continuously wound onto a rotating mandrel. The circumferential velocity of the mandrel is generally higher than the velocity by which the fiber is extruded so that considerable stretching of the fiber takes place during winding. Because the fiber is still hot (melt processing) or still contains solvent (solution processing) when it reaches the mandrel, fiber-fiber binding takes place. After a number of passes, the desired thickness is reached. The fibrous structure may then be dried, cured, cooled, and removed from the mandrel. A porous, stable tube can result. The use of such a continuous fiber winding technique to form a porous prosthesis has been described in Leidner et al., "A Novel Process for the Manufacturing of Porous Grafts: Process Description and Product Evaluation," J. of Biomedical Materials Res., Vol. 17, No. 2, March 1983, pp. 229-247, incorporated herein by reference.
Advantageously, the use of continuous fiber winding provides a prosthesis with a fibrous structure, which is very desirable in terms of performance (e.g. tissue ingrowth) and mechanical properties such as strength, compliance, flexibility, and the like. Unfortunately, continuous fiber winding techniques may only be used in connection with polymeric materials that are spinnable, e.g., good fiber formers. Yet, there are a host of polymer materials without such good fiber forming characteristics that nonetheless have other characteristics that are extremely desirable in the manufacture and subsequent use of prostheses. For example, silicone resins are a class of materials that are desirable in terms of strength, compliance, flexibility, biocompatibility, elasticity, and the like, but are not spinnable fiber formers. Consequently, silicone resins and similar materials generally are not compatible with the continuous fiber winding technique.
Electrostatic spraying is a technique that may be used to form a fibrous prostheses from a wide range of polymer materials, (including polymers such as silicone resins) that are otherwise poor fiber formers. According to this technique, a polymer melt, solution, or dispersion is extruded through a fine orifice and directed toward a rotating mandrel. A voltage is maintained between the orifice and mandrel so that the polymer material is attracted electrostatically to the mandrel. In practice, droplets of the polymer material extruded from the orifice are electrostatically pulled toward the mandrel. The mandrel is thus struck with a plurality of short polymeric fibers that eventually coat the mandrel. A desired thickness of material can be built up, after which the resultant prosthesis can be dried, cured, cooled, and removed from the mandrel.
Unfortunately, the conventional electrostatic spraying technique suffers from some drawbacks. In particular, the short polymeric fibers tend to coalesce after striking the mandrel, at least to some degree. This causes the inner wall of the resultant prosthesis to have low, if any, porosity. Silicone fibers, in particular, tend to coalesce when electrostatically sprayed onto a mandrel to such a degree that the inner wall of the prosthesis is substantially smooth. Thus, prosthesis produced by the electrostatic spraying of silicone tend to lack the degree of porosity that would facilitate the desired ingrowth of host tissue.
Accordingly, there is a need for an approach by which prostheses can be electrostatically sprayed from polymeric materials, particularly silicone fibers and other polymers that are poor fiber formers, in such a way that the prostheses have a beneficial degree of porosity on the inner wall surfaces.